Search for the $\tau \to \mu\mu\mu$ decay at CMS

The results of a search for the $\tau \to$ 3$\mu$ charged-lepton flavour violating decay in the CMS experiment are presented. The analysis is carried out on data collected during 2016 on proton-proton collisions at the centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 33 fb$^{−1}$. The analysis selects $\tau$ leptons produced in B and D meson decays. No evidence of signal is observed and an upper limit of 8.8×10$^{−8}$ is set on the $\tau$ → 3$\mu$ branching ratio at 90% of confidence level.